The present invention relates to an optical head for an optical-magnetic memory apparatus which uses a light beam for recording, retrieving, and erasing data. In recent years active research and development activities have been directed to an apparatus using a vertical magnetized film as a memory medium, and to He-Ne lasers, Argon lasers, semiconductor lasers, and other similar types of lasers for optically recording, retrieving and erasing. When this system is combined with magnetic disks, drums, tapes, and similar apparatus, it offers the major advantage of eliminating worry about the head becoming soiled from dust or other foreign objects, or about crushing of the head, as compared with conventional magnetic heads which record, retrieve, and erase by means of a magnetic head whose tip can be positioned about 1 mm from the memory medium.
In the usual optical memory apparatus where high density storage is possible, the dimensions of the record bit are about 1 .mu.m, and for this reason the optical beam must be squeezed into a minute spot. At the same time the size of the light spot must be kept uniform, making a servo-system necessary so that the abovementioned light spot may accurately be traced above the data bit. FIG. 1 shows an example of the basic configuration of the optical head section in a conventional apparatus using the diffraction effect of an optical beam to retrieve data from a data bit in a special retrival optical type video disk. In this system there is a laser apparatus 1, a polarized light beam splitter 2, a 1/4 wave length plate 3, an object lens 4, a memory disk 5, a spot lens 6, and an light detection apparatus 7. The said light detection apparatus 7, in order to receive a signal to control the position of the light spot takes various elemental forms. In addition, the said light detection apparatus 7, as well as receiving the abovementioned control signal, also receives data signals.
It is possible for the abovementioned type of conventional optical head used for an optical type video disk to be used in practice as an optical head on an optical magnetic memory apparatus. An example of this is given in FIG. 2. The optical elements which are equivalents to those shown in FIG. 1 carry the same number in FIG. 2 as in FIG. 1. The elements in FIG. 2 which are different than those for video disk usage because of the principles of data retrieval, are the providing of an analyzer 8 between the spot lens 6 and the light detection apparatus 7, the substitution of a polarizer 9 for the 1/4 wave length plate 3, and the substitution of a beam splitter with a half mirror function in place of the polarized light beam splitter 2. However, the problem with this configuration is that in optical magnetic retrieving a minute angle of rotation of the polarized light of the reflected data beam must be obtained. In short, when obtaining a data signal, because the transmission axis of the analyzer 8 must be set close to right angles to the direction of the polarized light from the reflected data beam, the quantity of light reaching the light detection apparatus 7, compared to the abovementioned optical type video disk, becomes much smaller. Then, because of this, it becomes extremely difficult to receive a sufficiently accurate control signal from the light detection apparatus 7 (focus error signal, tracking error signal).